This invention relates to a light beam scanning apparatus for causing, for example, laser beams to scan the surface of a photosensitive drum simultaneously to form an electrostatic latent image on the photosensitive drum and to an image forming apparatus, such as a digital copying machine or a laser printer, using the light beam scanning apparatus.
In recent years, various digital copying machines have been developed which form images by, for example, laser-beam scanning exposure and electronic photograph processing.
To step up the image forming speed, the multi-beam digital copying machines have recently been developed. In this type of digital copying machine, more than one laser beam is generated and they are caused to scan in units of lines simultaneously.
The multi-beam digital copying machine comprises semiconductor laser oscillators for generating laser beams, a polyhedral rotating mirror, such as a polygon mirror, and an optical system unit acting as a light-beam scanning apparatus. The polyhedral rotating mirror reflects the laser beams emitted from the laser oscillators toward a photosensitive drum to cause each laser beam to scan the surface of the photosensitive drum. The optical system unit is composed mainly of a collimator lens and an f-.theta. lens.
A method of controlling the exposure position accurately in the direction in which a laser beam scans (or the main scanning direction) in a digital copying machine of the multi-beam type has been disclosed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 1-43294, Jpn. Pat. Appln. KOKOKU Publication No. 3-57452, Jpn. Pat. Appln. KOKOKU Publication No. 3-57453, Jpn. UM Appln. KOKOKU Publication No. 5-32824, or Jpn. Pat. Appln. KOKAI Publication No. 56-104572.
Jpn. Pat. Appln. KOKOKU Publication No. 1-43294 has disclosed a method of using a light beam sensor to sense the timing with which light beams arrive. In this method, the order in which the light beams arrive is unknown. Therefore, the method is not suitable for an optical system where two or more light beam arrive simultaneously.
Jpn. Pat. Appln. KOKOKU Publication No. 3-57452 has disclosed a method of providing separate light-receiving sections for sensing light beams independently and permitting each light beam to expose the corresponding light-receiving section and pass through it. On the basis of the signal from each light-receiving section, the light-emitting timing for printing by each light beam (or recording or image formation) is produced.
However, for example, when plural recording dot pitches (resolutions), such as 300 dpi, 400 dpi, and 600 dpi, or 16 lines/mm and 15.4 lines/mm, are needed, the number of revolutions of the polygon mirror or the frequency of image clock must be changed. In this case, it is difficult to align the print start position of each light beam because of the following problems: the phase of the output signal from each light-receiving section may change with respect to the corresponding light beam, the timing with which each light beam arrives at the print start position, and the difference in arriving timing between the light beams cannot be divided by one period of image clock.
The method disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 3-57453 has been based on the assumption that the main scanning image formation area for each light beam is designed to shift in the main scanning direction. Therefore, the method is not suitable for such an optical system as is shown in embodiments of the present invention.
The method disclosed in UM Appln. KOKOKU Publication No. 5-32824 is not suitable for such an optical system as is shown in embodiments of the present invention because of the same reason as in the method in Jpn. Pat. Appln. KOKOKU Publication No. 3-57452.
The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 56-104572 is to produce a synchronizing signal using one of plural light beams and control the light-emitting timing for each light beam on the basis of the synchronizing signal. The relationship between the scanning positions of the light beams must be known beforehand. Therefore, the method is not suitable for such an optical system as is shown in embodiments of the present invention.